The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image areas and the water or fountain solution is preferentially retained by the non-image areas. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and other materials. Commonly, the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion that is exposed become hardened so that non-image areas are removed in the developing process. Such a plate is referred to in the art as a negative-working printing plate. Conversely, when those portions of the coating that are exposed become soluble so that they are removed during development, the plate is referred to as a positive-working plate. In both instances, the image areas remaining are ink-receptive or oleophilic and the non-image areas or background are water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance of positive-working plates, the areas on the film corresponding to the image areas are darkened, preventing light from making those areas developer soluble, while the areas on the film corresponding to the non-image areas are clear, allowing them to become soluble. The soluble image areas can be removed during development. The non-image surfaces of a positive-working plate remain after development, are oleophilic and will accept ink while the image areas that have had the coating removed through the action of a developer are desensitized and are therefore hydrophilic.
Various useful printing plates that can be either negative-working or positive-working are described, for example, in GB 2,082,339 (Horsell Graphic Industries), and U.S. Pat. No. 4,927,741 (Garth et al), both of which describe imaging layers containing an o-diazoquinone and a resole resin, and optionally a novolac resin. Another plate that can be similarly used is described in U.S. Pat. No. 4,708,925 (Newman) wherein the imaging layer comprises a phenolic resin and a radiation-sensitive onium salt. This imaging composition can also be used for the preparation of a direct laser addressable printing plate, that is imaging without the use of a photographic transparency.
Direct digital imaging of offset printing plates is a technology that has assumed importance to the printing industry. The first commercially successful workings of such technology made use of visible light-emitting lasers, specifically argon-ion and frequency doubled Nd:YAG lasers. Printing plates with high photosensitivities are required to achieve acceptable through-put levels using plate-setters equipped with practical visible-light laser sources. Inferior shelf-life, loss in resolution and the inconvenience of handling materials under dim lighting are trade-offs that generally accompany imaging systems exhibiting sufficiently high photosensitivities.
Advances in solid-state laser technology have made high-powered diode lasers attractive light sources for plate-setters. Currently, at least two printing plate technologies have been introduced that can be imaged with laser diodes emitting in the infrared regions, specifically at about 830 nm. One of these is described in EP 573,091 (Agfa) and in several patents and published applications assigned to Presstek, Inc. [for example, U.S. Pat. No. 5,353,705 (Lewis et al), U.S. Pat. No. 5,351,617 (Williams et al), U.S. Pat. No. 5,379,698 (Nowak et al), U.S. Pat. No. 5,385,092 (Lewis et al) and U.S. Pat. No. 5,339,737 (Lewis et al)]. This technology relies upon ablation to physically remove the imaging layer from the printing plate. Ablation requires high laser fluences, resulting in lower through-puts and problems with debris after imaging.
A higher speed and cleaner technology is described, for example, in U.S. Pat. No. 5,340,699 (Haley et al), U.S. Pat. No. 5,372,915 (Haley et al), U.S. Pat. No. 5,372,907 (Haley et al), U.S. Pat. No. 5,466,557 (Haley et al) and EP-A-0 672 954 (Eastman Kodak) which uses near-infrared energy to produce acids in an imagewise fashion. These acids catalyze crosslinking of the coating in a post-exposure heating step. Precise temperature control is required in the heating step. The imaging layers in the plates of U.S. Pat. No. 5,372,907 (noted above) comprise a resole resin, a novolac resin, a latent Bronsted acid and an infrared radiation absorbing compound. Other additives, such a various photosensitizers, may also be included.
DE-4,426,820 (Fuji) describes a printing plate that can be imaged in the near infrared at moderate power levels with relatively simple processing requirements. This printing plate has at least two layers: an imaging layer containing an o-diazoquinone compound and an infrared radiation absorbing compound, and a protective overcoat containing a water-soluble polymer or silicone polymer. This plate is floodwise exposed with ultraviolet light to convert the o-diazoquinone to an indenecarboxylic acid, which is then imagewise decarboxylated by means of heat transferred from the infrared radiation absorbing material. Development with an alkaline solution results in removal of areas not subjected to thermal decarboxylation. The pre-imaging floodwise exposure step, however, is awkward in that it precludes the direct loading of the printing plates into plate-setters.
Optical recording medium having laser imageable layers are described in U.S. Pat. No. 4,966,798 (Brosius et al). Such layers contain an infrared radiation absorbing dye or pigment in a phenolic resin, and are resident on a suitable polymeric support. Recordation is carried out using a laser to bring about a surface change in the imageable layer. Printing plates are not the same type of materials and require a different imaging process.
Thus, there is a need for simple printing plates that can be easily imaged in the near infrared at moderate power levels and require relatively simple processing methods.